The following description relates to hydraulic actuators and, more particularly, to a fail-fixed hydraulic actuator system that uses a stepper motor and a pilot valve with a nulling sleeve and a method of operating a fail-fixed hydraulic actuator system.
In many engine actuator applications, an actuator is sent into or positioned in a fail-safe position in an even of an electrical failure. This fail-safe position may be an extended or retracted position. In helicopters, however, the notion of automatically positioning an actuator in a fail-safe position instead of a last-commanded position in the event of an electrical failure might not be desirable because of a need to maintain certain flight control parameters. Indeed, in at least some cases, while it is actually desirable to hold the actuator in the last commanded position instead of the fail-safe position in the event of an electrical failure, the nature of control systems of typical hydraulically powered actuators of helicopters makes doing so difficult.